Carbon fiber is light and excellent in strength and modulus of elasticity and hence is widely used in sporting goods and leisure goods. In recent years, it has been more improved in performance characteristics and has begun to be used as a primary structural material for spaceships, airplanes, etc. It, however, is more expensive than conventional metal materials and the like as before, and therefore its spread to the fields of general industries and the manufacturing industry is slow and its application are limited to special purposes.
The basic reason of the expensiveness of carbon fiber is its low productivity, in particular, inefficient flameresisting treatment of precursor fiber for carbon fiber (hereinafter referred to as precursor fiber or merely as fiber). The flameresisting treatment of precursor fiber is an exothermic oxidation reaction and is accompanied by generation of a large quantity of heat. Therefore, when a rapid flameresisting treatment is carried out, a vigorous incontrollable reaction is induced by heat accumulation, so that the fiber is melted and cut and that a fire is caused in an extreme case. In order to avoid such a vigorous incontrollable reaction, flameresisting treatment is carried out usually over a period of at least about 1 hour to as long as several hours. This is a cause of a marked lowering of the productivity.
For reducing the flameresisting treatment time, Jap. Pat. Appln. Kokoku (Post-Exam. Publn.) No. 53-21396 (U.S. Pat. No. 4,065,549) has disclosed a process in which precursor fiber is brought into contact with the surface of a heater intermittently and repeatedly. However, when this process is employed, the precursor fibers tend to be fused together with one another and carbonization of the resulting flameresistant fiber cannot yield carbon fiber of practical use.
Further, Jap. Pat. Appln. Kokai (Laid-Open) No. 58-214525 (EP 100411) has disclosed a process in which precursor fiber is treated in a heated oxidative atmosphere while being brought into contact with cooling rollers intermittently. However, when this process is employed, the fiber on the rollers is not rapidly cooled because the temperature around the rollers is high. Moreover, depending on conditions, the fibers tend to be fused together with one another and a stable treatment is impossible because the residence time of the fiber in a heat treatment compartment is not limited to 60 seconds or less, unlike in the present invention.
Still further, Jap. Pat. Appln. Kokoku (Post-Exam. Publn.) No. 51-9410 has disclosed a process in which a zone where fiber is heat-treated and a zone where rollers are accommodated are isolated from each other and the fiber is treated while keeping the temperature of the rollers and the temperature of an atmosphere in the zone where rollers are accommodated lower than the temperature of the zone where fiber is heat-treated. However, also in this process, the residence time in a heat treatment compartment is not limited to 60 seconds or less, unlike in the present invention. Therefore, a stable treatment is impossible depending on conditions, and the fiber is cooled too much, resulting in delay of its reaction in the next heat treatment compartment, because the temperature of the rollers and the temperature of the zone where rollers are accommodated are kept lower than 180.degree. C. Consequently, reduction of the flameresisting treatment time becomes difficult in some cases.
In addition, DOS2026019 has disclosed a process in which fiber is treated by providing rollers outside a furnace in order that the temperature of the rollers is lower than the fusing temperature of the fiber. This process, however, has the same defect as described above because the residence time in a heat treatment compartment is similarly not limited to 60 seconds or less, unlike in the present invention.